Field of the Invention
Exemplary aspects of the present disclosure generally relate to a fixing device and an image forming apparatus, and more particularly, to a fixing device for fixing a toner image on a recording medium and an image forming apparatus including the fixing device.
Description of the Related Art
Related-art image forming apparatuses, such as copiers, facsimile machines, printers, or multifunction printers having at least one of copying, printing, scanning, and facsimile capabilities, typically form an image on a recording medium according to image data. Thus, for example, a charger uniformly charges a surface of an image bearing member (which may, for example, be a photosensitive drum); an optical writer projects a light beam onto the charged surface of the image bearing member to form an electrostatic latent image on the image bearing member according to the image data; a developing device supplies toner to the electrostatic latent image formed on the image bearing member to render the electrostatic latent image visible as a toner image; the toner image is directly transferred from the image bearing member onto a recording medium or is indirectly transferred from the image bearing member onto a recording medium via an intermediate transfer member; a cleaning device then cleans the surface of the image carrier after the toner image is transferred from the image carrier onto the recording medium; finally, a fixing device applies heat and pressure to the recording medium bearing the unfixed toner image to fix the unfixed toner image on the recording medium, thus forming the image on the recording medium.
Known fixing devices employ a belt-type fixing member (hereinafter referred to simply as fixing belt) to fix an unfixed toner image onto a recording medium such as paper and an OHP film. In order to facilitate an understanding of the novel features of the present invention, as a comparison, a description is provided of conventional fixing devices with reference to FIGS. 9 and 10. As illustrated in FIG. 9, an example of such a fixing device using a fixing belt includes a looped belt 100, a tubular metal thermal conductor 200 disposed inside the loop formed by the belt 100, a heat source 300 disposed inside the metal thermal conductor 200, and a pressing roller 400 that contacts the metal thermal conductor 200 via the belt 100 to form a nip portion N. The heat source 300 in the metal thermal conductor 200 heats the belt 100 through the metal thermal conductor 200.
Another example of the fixing device using the fixing belt heats the belt directly without the metal thermal conductor as illustrated in FIG. 10. As illustrated in FIG. 10, the fixing device includes the belt 100 without the metal thermal conductor, but instead, a planar nip forming member 500 is disposed opposite the pressing roller 400. The heat source 300 is disposed inside the looped belt 100. In this configuration, the heat source 300 can heat a certain area of the belt 100 at which the nip forming member 500 is disposed, but other areas of the belt 100 as well, thereby increasing significantly heat transfer efficiency and hence reducing power consumption. Accordingly, a first print time from a standby state can be shortened.
To shorten a warm-up time and the first print time, the fixing belt may be made thin. However, the thin belt does not have good thermal conductivity and the belt is not heated uniformly when heated by the heat source. More specifically, in order to enhance the heat transfer efficiency, the fixing device may employ a reflective plate to concentrate heat to specific areas of the fixing belt. In this configuration, when heated, the specific areas of the fixing belt are heated selectively and the temperature thereof is higher than that of other areas. In a case in which a plurality of heat sources are employed, specific areas of the fixing belt at which the heat generating portions of the heat sources overlap in a longitudinal direction (a direction perpendicular to a sheet moving direction) are heated more than other areas.
If the temperature of the fixing belt is partially high, thermal expansion of that place is greater than that of other areas, causing deformation of the fixing belt. More specifically, the fixing belt expands outward.
Generally, the fixing device using the fixing belt employs a temperature detector for detecting the temperature of the fixing belt. However, when the fixing belt expands outward as described above, relative positions of the fixing belt and the temperature detector change, causing inaccurate detection of the temperature of the fixing belt.
In view of the above, there is demand for a fixing device capable of detecting the temperature of a fixing belt accurately even when the temperature thereof is high, and an image forming apparatus including the fixing device.